1. Field of the Invention
The present invention relates to a liquid developer used for image formation, and particularly relates to an electrophotographic liquid developer which is used for developing electrostatic latent images in image forming apparatuses such as electrophotographic copying machines and printers.
2. Description of the Background Art
Electrophotographic image formation is generally performed as follows. An electrostatic latent image is formed, e.g., by effecting image exposure corresponding to an original image on an electrostatic latent image carrier such as a photosensitive member. The electrostatic latent image thus formed is developed into a visible toner image. This visible image is transferred onto and fixed on a record member so that an intended image is formed.
In the electrophotographic image formation, the developing method is classified into a dry developing method and a wet developing method.
In the dry developing method, a developer is formed of toner (coloring fine particles), or is formed of the toner and carrier which has magnetic properties and is added to the toner. If the dry toner used for dry development were excessively fine, the toner would flow into an ambient atmosphere and would float therein. Therefore, the toner which is usually used has a relatively large average particle diameter of about 10 .mu.m or less.
In the wet developing method, the liquid developer which is now in the mainstream is formed of electrically insulating carrier liquid, in which toner primarily made of a coloring agent and binder resin as well as a charge director, a dispersion stabilizing agent and others are dispersed. It has been considered that the toner is charged owing to absorption of ions by virtue of the charge director, and the charged toner is used for development on the principle of electrophoresis. Since there is no possibility that the toner used in the wet development escapes into the atmosphere, fine toner can be used, and the available average particle diameter may be of the order of submicrons. Thereby, the produced image can have a high resolution and a good gray scale property.
An electrophoretic moving speed of the toner depends on the quantity of charges carried by the toner, and the developing speed can be increased with quantity of charges. As the quantity of charges increases, the dispersibilities of toner are improved, and the preservation properties of the liquid developer are improved. For improving the speed and stability of the wet developing system, therefore, it is necessary that the toner has a large quantity of charges.
With the increasing use of personal computers and others, it has been demanded that full-color images can be formed or duplicated by simple operations.
For forming the full-color image, offset printing has been employed as a method for formation or duplication of fine and beautiful full-color images. The offset printing allows fast production of high-quality images with a low cost, and therefore is suitable to large-volume printing. However, maintenance and operation of the offset printing apparatus are difficult.
In contrast to the above, electrophotographic copying machines, printers and others have such advantages that flexible image formation can be performed with easy operation. Compared with other printers of an ink-jet type or the like, the electrophotographic device is superior in printing speed, reproducibility and durability.
Year after year, higher qualities of full-color images have been required. For obtaining the high image quality substantially equal to or higher than that of the offset printing, it is suitable to employ the wet developing method which uses the toner of a small particle diameter and high chargeability.
For formation of the full-color image, precise control of the weight of toner developer is requiread in contrast to the mono-color image formation, and therefore it is necessary to stabilize the charge quantity of toner more sufficiently. Also, the high charge quantity is required for obtaining fine images of high resolutions.
For the above purpose, such a technique has been used in some cases that a charge control agent (CCA) is added to the dry developer for adding a high chargeability to the toner. As such charge control agent are known a salicylic acid metal salt and its derivative. The salicylic acid metal salt and its derivative provide negative charge to a toner. Since the salicylic acid metal salt and its derivative are white or colorless, they have no problem in providing a desired color to the toner and are suitable as a charge control agent for color developer. For example, Japanese patent No. 2625106 discloses a toner for dry developer containing the salicylic metal salt or its derivative as a charge control agent.
Further, a borate-containing compound has been known as the charge control agent. The borate-containing compound applies negative charges to the toner. Since many kinds of borate-containing compounds are colorless or light in color, they do not color the toner, and are suitable as the charge control agent for the developer of color images. For example, Japanese Patent Publication No. 8-10361 discloses the toner for dry development, which includes the borate-containing compound as the charge control agent.
However, if the salicylic acid metal salt or its derivative is used as the charge control agent for the toner in the liquid developer, such metal salt or its derivative is likely to dissolve in carrier liquid, i.e., organic solvent, and characteristics of the liquid developer may change with time. For example, dissolving of salicylic acid metal salt or its derivative lowers the chargeability of toner, and an ingredient dissolved in the carrier liquid causes lowering of the electric resistance of the carrier liquid or the like. For example, these disadvantages result in lowering of the developing speed, variations in image density, occurrence of image failures such as blur and/or dispersion in image, and condensation of toner during preservation.
If the borate-containing compound is; used as the charge control agent of the toner in the liquid developer, it is likely to dissolve in the carrier liquid (organic solvent) so that the characteristics of liquid developer may change with time. For example, dissolving of borate-containing compound causes lowering of the chargeability of toner, and an ingredient dissolved in the carrier liquid causes, for example, lowering of the electric resistance of the carrier liquid. These disadvantages result in, e.g., lowering of the developing speed, variations in image density, occurrence of image failures such as blur and/or dispersion in image and condensation of toner during preservation.
For forming the full-color images, developers of four colors, i.e., yellow, magenta, cyan and black are used similarly to the offset printing and others, and images each formed with the developer of the corresponding color are overlapped with each other by a subtractive color mixture method. If these developers of four colors are of a dry type, each developer can be produced by adding appropriate coloring pigment or dye as well as various additives, if necessary, to binder resin in a dispersed fashion, and thereby providing toner of a predetermined particle diameter. In the case of the liquid developer, the developer can be produced by dispersing such toner in the carrier liquid.
Among the toners of respective colors, the yellow toner is primarily formed of Color Index Pigment Yellow (which will also be referred to as "PY" hereinafter) 12, PY13, PY17, PY174, PY176 or the like, which has been widely used in conventional printing ink and others, because these pigments are preferable in view of coloring properties and cost.
However, the above Pigment Yellow contains organically bonded chlorine and heavy metal in the molecule. Therefore, it is proposed to use PY180 or Color Index Solvent Yellow (hereinafter may be referred to as SY) 162,. each of which is an azo compound containing no organically bonded chlorine and heavy metal and has excellent color forming ability almost same as conventional Pigment Yellow. PY180 is an azo pigment which has been known under the trade names of Novoperum-Gelb P-HG, Toner Yellow HG VP2155 and others. PY180 and dry toner containing the same are disclosed in U.S. Pat. Nos. 4,870,164 and 4,935,502, European patent application No. EP 0 705 886 A2 and others. SY162 is an azo dye which has been known under the trade names of Neopon Yellow 075 and others. SY162 and dry toner containing the same are disclosed in Japanese Laid-Open Patent Publication Nos. 61-112160, 7-140716 and 8-234490, and others.
However, if the above PY180, SY162 or derivative thereof is used as a coloring agent in the liquid developer, these are more likely to dissolve in the carrier liquid than the conventional yellow pigment. Depending on the types of the coloring agent and carrier liquid, therefore, characteristics of the liquid developer may change with time. For example, dissolving of the yellow coloring agent causes lowering of the chargeability of toner, and an ingredient dissolved in the carrier liquid causes lowering of the electric resistance of the carrier liquid or the like. These disadvantages result in lowering of the developing speed, variations in image density, occurrence of image failures such as blur and/or dispersion in image, condensation of toner during preservation and others.
In the liquid developer, the content of the coloring agent with respect to the binder resin in the toner is larger than that of the dry developer, and therefore the coloring agent affects the chargeability of toner to a higher extent. The toner of the liquid developer, of which coloring agent is formed of the foregoing PY180, SY162 or the derivative thereof, has a lower chargeability than the toner containing conventional yellow pigment, and therefore cannot achieve a practically required high developing speed without difficulty. In the liquid developer, the toner has a small particle diameter, and therefore a high toner chargeability is required for obtaining an intended developing speed. In the liquid developer using the foregoing PY180, SY162 or the derivative thereof as the toner coloring agent, it is unavoidable to increase the toner particle diameter for obtaining the intended toner chargeability, which results in a problem that high-resolution images cannot be formed.
If the foregoing PY180 is used as the coloring agent of the toner in the liquid developer, the pigment cannot disperse in the binder resin to an intended extent so that image failures such as blur or loss of color are likely to occur in final images. The PY180 may be directly dispersed in the carrier liquid without using the binder resin. In this case, however, PY180 cannot be dispersed in the carrier liquid to an intended extent, and is likely to be condensed or deposited.